Recent manufacturing techniques for computer chips require removal of a large portion of the base layer of silicon after the computer chips have been built up on the base layer. To remove the base layer, a wafer with numerous devices built up on the wafer is thinned in a process referred to as backgrinding. Backgrinding is accomplished after the devices are built up, but before the wafer is diced into individual computer chips. The goal of the backgrinding is to reduce the thickness of the base layer of silicon to a predetermined thickness. When the backgrinding process is used to reduce the thickness of the base wafer to such a thin dimension that the wafer is extremely weak, and is subject to easy breakage, the base wafer is sometimes mounted to another full-thickness wafer to provide handling strength. The wafer to be ground thin is attached to a carrier wafer by adhesive, by van der Waals forces, or other means, creating a stack of wafers. The thin wafer can then be processed with much less chance of breakage. The process is used for production of back-side illumination chips (for cameras), through-silicon vias, and 3-D vertically integrated circuit packages.
During grinding operations of single, non-stacked wafers, thickness is usually measured with a contact probe. The contact probe references the surface of the grind chuck prior to chucking the wafer. During the grinding, the contact probe maintains contact with the ground surface, permitting continuous monitoring of the wafer thickness. Contact probes are not always suitable for use with stacked wafers because they do not provide separate information of the top wafer thickness. Variations in carrier wafer thickness can lead to erroneous top wafer thickness measurement by the contact probe.
Non-contact probes that employ optical techniques, such as interferometry, can be used to measure the thickness of stacked wafers. However, during the grinding process, debris builds up on the back surface of the wafer so that the light emitted by the probe is blocked by the debris.